1. Field of the Invention
The present invention relates to a high-frequency matching circuit, and, more particularly, to a short-stub matching circuit in a monolithic integrated circuit, which is used in the millimeter wave band and microwave band.
2. Description of the Related Art
In designing millimeter wave/microwave amplifiers and making their prototypes, parasitic oscillation generally often worries the designers. This is because a highfrequency transistor to be used as an amplifier element. e.g., a heterojunction FET, has a power gain of greater than 0 dB over a wide frequency range from DC (direct current) to a microwave/millimeter wave band, the circuit meets the oscillation condition in some frequency band.
The stability condition for a transistor is such that with a variable load impedance Z1 connected to the output terminal of the transistor, the absolute value of a reflection coefficient in on the input side is smaller than "1" even when the variable load impedance Z1 lies anywhere within the Smith chart. In other words, no oscillation occurs when the absolute values of the input reflection coefficient in and an output reflection coefficient out are both smaller than "1."
In general, a stability coefficient K, an index, is used to determine whether or not oscillation will occur. Stability can be discriminated by checking if the value of the stability coefficient K is greater than "1." The stability coefficient X is given from the following equation using S parameters of the circuit. EQU K=(1-.vertline.S.sub.11.vertline..sup.2 -.vertline.S.sub.22.vertline..sup.2 +.vertline..DELTA..vertline..sup.2)/2.multidot..vertline.S.sub. 12.multidot.S.sub.21.vertline..DELTA.=S.sub.11.multidot.S.sub.22 -S.sub.12.multidot.S .sub.22 -S.sub.12.multidot.S.sub.21.multidot.
The condition for absolute stability is K&gt;1 in which case no oscillation occurs with respect to every passive load impedance Z1 unless a feedback circuit Is externally added.
In Japanese Patent Unexamined Publication (Kokai) No. 7-240369, a matching circuit has been disclosed which is designed in consideration of the low-frequency stability in an amplifier circuit. This matching circuit is connected to a radio-frequency (RF) signal transfer line which is connected to the gate electrode of an FET (field effect transistor), and serves as both a bias circuit and a stabilizing circuit.
More specifically, the matching circuit has a .lambda./4 transfer line connected to the RF signal transfer line, and the .lambda./4 transfer line is grounded via a first MIM (Metal-Insulator-Metal) capacitor, and is further grounded via a second MIM capacitor and a resistor connected in series, where .lambda. is a wave length. That is, the first MIM capacitor and the second MIM capacitor and the resistor form a parallel circuit.
The capacitance of the first HIM capacitor is set in such a way that the first MIM capacitor becomes nearly short-circuited at the use frequency and becomes nearly an open state at a low frequency outside the use frequency. The capacitance of the second MIM capacitor is set in such a way teat it is greater than the capacitance of the first MIM capacitor and the second MIM capacitor becomes nearly short-circuited at a low frequency outside the use frequency. Therefore, the matching circuit serves as a bias circuit with the .lambda./4 transfer line grounded via the first MIM capacitor at the use frequency, and serves as a stabilizing circuit for prevention of oscillation, with the .lambda./4 transfer line grounded via the series-connected second MIM capacitor and resistor at a low frequency outside the use frequency.
In Japanese Patent Unexamined Publication (Kokai) No. 1-233812, another matching circuit has been disclosed which has an oscillation preventing resistor inserted on the opposite side to the short-circuit side of a short-stub for matching. This matching circuit aims at decreasing the number of elements in an oscillation preventing circuit in a monolithic integrated circuit (IC), thereby reducing the chip area. To achieve this purpose, the oscillation preventing resistor is directly formed in the RF signal transfer line.
In the matching circuit disclosed in Unexamined Patent Publication (Kokai) No. 7-240369, however, as shown in FIG. 14, there is an area 1 where the stability coefficient K becomes K&lt;1 at a low frequency near 10 GHz. This matching circuit thus does not satisfy the complete stability condition.
Because the matching circuit disclosed in Unexamined Patent Publication (Kokai) No. 1-233812 has the oscillation preventing resistor directly formed as a semiconductor resistor in the RF signal transfer line, the degree of freedom at the time of implementing trimming in the fabrication process of a microwave monolithic IC (MMIC) is significantly limited.